Equivocation

Equivocation is a logical fallacy that involves taking a word with more than one definition and freely substituting one definition for another.

For example: "A feather is light. Therefore, a feather cannot be dark." There are two meanings of the word "light." The first sentence assumes a meaning that is the opposite of "heavy," not the opposite of "dark."

This fallacy is used frequently in the service of apologetics arguments. A few examples: